What is the next frontier in mobile UX/UI design?
Three things that are going to be a big focus for mobile UX/UI designers in the years to come:
Voice Interaction: Accessing information from your mobile device or the cloud through speech is here now, but we "turn it on" to do so. I expect this to change in the future, and that your mobile phone to always be "on call" and "wake up" when spoken too. I also expect the information and functions that will be accessed through voice commands to become more complex, useful and proactive: think "digital butler." Not a lot of "UI design" here, but a lot of UX design thought needs to go into making these experiences work seamlessly and well.
Smart Devices: See the "digital butler" note above, and think "service design" for end-users… smart devices that learn suggest and help (sending a text to someone if you are running late for a meeting, for example.) Again, more process and AI workflows than UI.
Extending and/or tethering: Think "wearable computers", with devices like the Pebble wristwatch that connects to your mobile phone via low-powered bluetooth, allowing the user to make calls or see messages on his or her wrist. Also think about how mobile devices can engage with devices or locations in your home (opening a grocery list app when you stand next to your pantry for a long while, or "talking" to your smart TV while you are watching a show). Lots of potential UIs that need designing here…